Diesel engines belong to the category of lean-bum engines and are operated with lean air/fuel-ratios. The air/fuel-ratio is calculated from the mass of air supplied to the engine in relation to the mass of fuel. In normal fuels for internal combustion engines such as diesel engines or gasoline engines, 14.6 kilograms of air are needed for the complete combustion of 1 kilogram of fuel, which is an air/fuel-ratio of approximately 14.6. Air/fuel-ratios above that value are lean, and air/fuel-ratios below that value are rich. The exhaust gas leaving the engine exhibits the same air/fuel-ratio as the air/fuel-mixture supplied to the engine, provided that no adsorption or desorption processes occur within the engine.
Frequently the composition of the air/fuel-mixture or of the exhaust gas is characterized by a lamda (λ) value. λ is defined as the air/fuel-ratio normalized to stoichiometric conditions. For stoichiometric combustion of the fuel, the λ-value of the air/fuel-mixture supplied to the engine must be equal to 1.
Diesel engines are operated with lean air/fuel-mixtures with λ-values above 1, usually with λ-values of between 1.5 and 4. The exhaust gas of diesel engines contains a high oxygen concentration of between 5 and 15 volume-%, compared to gasoline engines, which contain only approximately 0.7 volume-% oxygen.
Diesel engine exhaust gases contain harmful substances such as: carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), and soot particles. The nitrogen oxides are a mixture of different oxides of nitrogen. The major component is nitrogen monoxide, which is from 60 to 90 volume-% of the total nitrogen oxides content of the exhaust gas; the balance is mainly nitrogen dioxide. The exact composition depends on the engine type and the operating conditions.
Carbon monoxide and unburned hydrocarbons can be effectively converted to harmless substances by contacting the exhaust gas with a diesel oxidation catalyst. Unfortunately, due to the high oxygen content of diesel exhaust gas, it is difficult to convert the nitrogen oxides to harmless nitrogen gas. For coping with this problem, nitrogen oxide storage catalysts have been developed that adsorb nitrogen oxide during lean operating phases, then release nitrogen oxides and convert them to harmless substances during rich exhaust gas operating phases.
Nitrogen oxide storage catalysts are composed of mainly a platinum catalyst and a storage component. The storage component is usually a basic metal oxide, such as an oxide of an element selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and mixtures thereof. Preferred storage components are barium oxide and strontium oxide.
The accepted theory of nitrogen oxide storage catalyst function is as follows. During lean operating phases, nitrogen monoxide contained in the exhaust gas is oxidized by the platinum catalyst to nitrogen dioxide. Under the humid atmosphere of the exhaust gas, nitrogen dioxide is trapped by the storage component in the form of nitrates. When the storage capacity of the nitrogen oxide storage catalyst has been exhausted, it needs to be regenerated to restore its original storage capacity. Regeneration is achieved by changing the air/fuel-ratio of the air/fuel-mixture fed to the engine to rich values. Rich exhaust gas establishes reducing conditions under which adsorbed nitrogen oxides are desorbed, and with the help of carbon monoxide and hydrocarbons contained in the rich exhaust gas, the nitrogen oxides are converted to nitrogen, carbon dioxide and water by the platinum catalyst.
The storage phase is defined as the time during which the nitrogen oxide storage catalyst can adsorb nitrogen oxide, and is usually between 1 and 5 minutes, depending on the nominal storage capacity of the nitrogen oxide storage catalyst and the concentration of nitrogen oxide in the exhaust gas. After the storage phase, the catalyst needs to be regenerated. Regeneration is accomplished by lowering the λ-value of the exhaust gas to between 0.9 and 0.95. From between 5 and 10 seconds of regeneration time is needed to restore the catalyst storage capacity. Thus, storage and regeneration alternate frequently during operation of the engine.
Diesel engines require lean air/fuel-mixtures for stable operation. It was only with the recent development of new diesel engines such as common rail engines and pump-injector engines, that it became possible to operate diesel engines with rich air/fuel-mixtures for a short period of time. This development made it possible to use nitrogen oxide storage catalysts for exhaust gas cleaning diesel engines, as well as lean operated gasoline engines.
Changing the air/fuel ratio from lean to rich to regenerate the nitrogen oxide storage catalyst during driving must be performed in such a way that does not affect the driving comfort. This restricts the maximum permissible regeneration time to approximately 8 to 20 seconds. This time period is sufficient to regenerate the nitrogen oxide storage catalyst completely, provided that the exhaust gas temperature is high enough.
Nitrogen oxide storage catalyst regeneration functions well above a threshold temperature between approximately 170° C. and 250° C., but regeneration is difficult below this temperature range. Newly developed diesel engines exhibit relatively low average exhaust gas temperatures. This causes problems with regenerating nitrogen oxide storage components, especially after prolonged storage periods. The conventional regeneration procedure involves brief rich periods during which the stored nitrogen oxide is released and subsequently converted to nitrogen. However, at low temperatures a dsubstantial amount of the nitrogen oxide being released leaves the converter unreduced, probably due to slow kinetics of the chemical reactions involved in nitrogen oxide conversion. Furthermore, the storage components are only partly cleared, and some nitrates remain in the storage material, lowering the storage capacity for the next storage cycle. The situation is aggravated even further by HC and CO breakthroughs during the rich phase, common under these conditions. Heating measures in lean conditions by post injection are ineffective at improving the performance of nitrogen oxide storage catalysts, since the temperature increase achieved drops very quickly after switching back to normal operation mode.
Therefore, a need exists for improving the regeneration of nitrogen oxide storage catalysts, especially at low exhaust gas and catalyst temperatures.